1. Field
The present disclosure relates to air shrouds for nozzles, more specifically to air shrouds for fuel nozzles such as in gas turbine engine fuel injectors.
2. Description of Related Art
Fuel nozzles allow for mixing of fuel and air for injection into a combustor. Due to the turbulent nature of the flow-field, some of the liquid fuel spray from the fuel nozzle will wet the metal surfaces of the fuel nozzle which are exposed to the hot combustion gases. If the fuel temperature on the surface of the metal is in the proper range (about 200° C. to about 400° C. for jet fuel), then fuel will chemically break down to form carbon deposits on the metal surfaces. This can occur on the exposed surfaces of fuel pre-filmers and/or air-caps (also called air-shrouds). Carbon-formation on these metal surfaces is undesirable because this can adversely affect spray and combustion performance. Also, this carbon can sometimes break free from the metal surface and flow downstream where it can come into contact with the turbine and cause turbine erosion, which shortens the life of the turbine. In other cases, the exposed metal surfaces of the fuel nozzle (most commonly the air-shrouds) are subject to excessive heating from the combustion gases, which can result in thermal erosion or cracking of the metal.
A common method to alleviate either the problem of carbon-formation or thermal-erosion is to add an additional (smaller) air-shroud outboard of the existing air-shroud. This smaller air-shroud is commonly called an air-wipe and serves the function of directing compressor-discharge air downward over the face of the first (larger) air-shroud to either preferentially prevent carbon-formation or alleviate thermal-erosion. In some cases, these air-wipes also experience thermal-erosion and require some method to manage the thermal load. Typically, a series of small holes through the air-wipe are added to provide additional cooler compressor-discharge air in order to reduce the thermal load. Often this will alleviate the problem, but not always. In some cases, it is difficult to get a sufficient amount of additional compressor-discharge air in the vicinity of the air-wipe. In other cases, the thermal loading results in differential thermal expansion of the air-wipe which results in cracking and reduced life of the fuel nozzle, or possible damage to the turbine due to the air-wipe liberating from the fuel nozzle and traveling downstream through the turbine. Therefore, there is still a need in the art for improved air-wipes. The present disclosure provides a solution for this need.